The modern agricultural industry has devoted considerable resources toward the development of phenotypically distinct plants with economically advantageous qualities. Valuable features in food crops include increased vigor, disease resistance, greater yields, extended shelf-life, and enhanced nutritional content.
The development of high yielding food crops is particularly important. Each year, the tillable land available for agricultural production is reduced as more acreage is devoted to alternative uses. At the same time, the human population is rapidly increasing. Therefore, it is essential to increase agricultural productivity in order to meet the nutritional needs of the world's burgeoning population.
Efforts to develop crop plants that produce higher yields have been directed toward pest control, or toward the selection and breeding of varieties that bear greater numbers of fruit, or that produce larger fruit. These crop breeding endeavors are very time-consuming and labor-intensive, but have historically increased crop yields incrementally over time. Modern techniques of recombinant DNA manipulation and genetic engineering offer the prospect of the more rapid creation of new plant varieties with novel traits. The creation of genetically modified, or transgenic, plants with altered phenotypes arising from artificially inserted genetic constructions has become a common practice in modern agriculture.
If one is going to genetically engineer plants, the genetic engineering or recombinant DNA manipulation of plastids is one area in which improvements to plants might be targeted. Plastids are membrane-delimited organelles in plant cells which are essential for sustaining plant growth and cell viability. They are the site for the synthesis of essential amino acids, vitamin E, pro-vitamin A, starch, certain growth hormones, lipids, and pigments such as carotenes, xanthophylls, and chlorophylls. In plants, plastids include chloroplasts, chromoplasts, leucoplasts and amyloplasts, which are typically found in all organs of the plant including its leaves, roots, stems, petals, and seeds.
The specialized plastid chloroplast is where photosynthesis occurs. Photosynthesis in plants is an important biosynthetic process upon which virtually all living organisms depend for our very existence. During photosynthesis, energy in the form of light is converted to ATP, which fuels a series of enzymatic reactions that catalyze the synthesis of carbohydrates, which are further used for metabolic energy in the plant. Photosynthesis also produces molecular oxygen (O2) is a byproduct. Because photosynthesis is the source of metabolic energy in plants, photosynthetic efficiency is a significant factor associated with general plant growth and vigor. Chloroplasts also synthesize amino acids and lipids.
U.S. Pat. No. 5,981,836, incorporated herein by reference, discloses genetic constructs capable of altering the number and size of plastids in plant cells. These construct contain an Arabidopsis plastid division FtsZ protein coding sequence and a promoter, not natively found associated with the FtsZ protein coding sequence, which promotes expression of the Arabidopsis plastid division FtsZ protein coding sequence in the plant. The FtsZ protein is a bacterial cytoskeletal protein and structural homologue of tubulin that polymerizes on the inner surface of the cytoplasmic membrane to forms a cytokinetic ring during cell division. Transgenic expression of the coding sequence results in a high percentage of novel phenotypes characterized by alterations in the number and size of plastids in the cells of the plant in which the construct is expressed.
There is also on-going efforts to make transgenic plants that are more suited for particular applications or which have transgenes inserted into them to have localized effects inside the cells of the plants. For example, there are a number of transgenes inserted into plants which maximize the usefulness of the inserted traits if the transgenes are transformed into the chloroplasts of the plant. Since one method for chloroplast transformation is based on the delivery of transgenes coated onto small carrier particles into the interior of the chloroplasts themselves, this technique is easier to perform if the chloroplasts themselves are larger than normal. So one technique that would be useful for this effort is to make plants with larger chloroplasts.
What is needed in the art are additional means for altering the shape, size and/or number of chloroplasts and other plastids in agronomically and horticulturally important plants to achieve greater plant productivity and nutritional quality.